The official blog of University of Missouri Skeptics, Atheists, Secular Humanists, & Agnostics

A Poorly Illustrated Guide to the Tree of Life: Part 3

Hello again. My name is James, and I like science. It’s been a while, but I’m back with some more words and drawings to tickle your every fancy. Our gradual climb up the Tree of Life continues. Where it will lead, none can be certain. What we can be certain of, however, is this: you’re about to become uncomfortably familiar with one of the Animal Kingdom’s most wondrous creatures: the Ctenophore. These mysterious undersea jellies may not be as cuddly as a kitten or a bunch of dudes with beards, but don’t fret, Dear Reader. By the time you finish this article, I promise you’ll discover that you’ve been in love with Comb Jellies all along.

Part 3.1: The Ctenophore

Comb jellies are ocean-dwelling invertebrates belonging to the phylum Ctenophora (it’s roughly pronounced “ten-o-four;” the ‘C’ is silent for some reason I can’t even begin to understand), which is Greek for comb-bearer. They come in a variety of shapes and sizes: some are inconspicuous specks of plankton, while others are brightly colored, downright gaudy little deep-sea vixens. The average Ctenophore is a transparent aquatic oblong with several rows of hair and a pair of long tentacles. If you’re unfortunate enough to have never seen one of these majestic beasts before (or you’re too lazy to look on Wikipedia), kindly observe this image painstakingly constructed with the latest rendering technology (i.e. Mac Paintbrush).

Behold!

Although Ctenophores are a strictly marine taxon, with no known freshwater species, they’re an extremely adaptable bunch. Comb jellies can be found near the coasts, in the middle of the open ocean, inside shallow inland seas, and in the crushing abyssal depths. Their versatility is a result of their simple but elegant design. The following section contains a brief account of the basic anatomy and physiology for an idealized Ctenophore, but be warned, Dear Reader; there are many variations and exceptions to these descriptions, some of which I’ll address in later on.

Anatomy of a Deep Sea Degenerate

Comb jelly. Get it? He's got a greaser hairstyle, because the ocean is just like the 1950s.

Ctenophores have partial bilateral symmetry, with a distinct top and bottom but no front or back. The top bears the entrance to the creature’s gastrovascular system (i.e. the mouth) and is designated as the oral surface. The bottom, known as the aboral surface, houses a set of anal pores and a sensory structure called the statocyst. Each jelly is equipped with 8 rows of cilia combs, or ctenes, that extend along the length of their bodies from near the mouth into the statocyst. These cilia move in a rhythmic motion that propels the jelly anywhere it fancies while simultaneously producing dandy little moving rainbows across their surface.

The statocyst, which is itself comprised mainly of 4 stiff tufts of cilia, provides the jelly with information about its position relative to the pull of gravity. Each of the tufts is physically contiguous with 2 neighboring ctenes and supports a small grain of calcium carbonate, the statolith, in the center of the statocyst. The whole structure works sort of like a simplified human inner ear: when the comb jelly is perfectly vertical with respect to gravity (looking up or down for you non-sciency types), the statolith exerts equal pressure on all 4 tufts; whenever the jelly is tilted horizontally (like your mom after a night of appletinis and cooking wine), the pressure in increased on the lower tufts and decreased on the upper tufts. The variations in pressure are translated into electrical signals which pass into the Ctenophore’s nerve net and are interpreted by what can only be described as magic. Neurobiology isn’t my forte.

A very realistic close-up of a statocyst.

Just like sponges, the bulk of a Ctenophore’s volume is composed of water suspended in a gel-like matrix of proteins, the mesoglea, sandwiched between sheets of epithelium. Unlike sponges, comb jellies have fewer cells embedded in their middle layer and possess unambiguous and persistently interconnected tissue layers, although there is some disagreement in the scientific literature as to whether there are 2 or 3 layers (some authors have squared this circle by positing that comb jellies have 2.5 epithelial layers. Good job, Science).

Additionally, Ctenophores have fibrous myoepithelial (i.e. muscle) cells, a feature that may shed light on the evolution of the middle tissue layer which gives rise to muscle tissue in triploblastic animals like me and you, Dear Reader. Cells composing the outer surface of the jelly form the epidermis, which functions to protect and provide support to the animal’s interior bits. The epidermis also houses a network of neurons that act as the Ctenophore’s nervous system.

A Neapolitan ice cream representation of Ctenophore tissues. Strawberry indicates the outer ectoderm layer, with the yellow squiggles (butterscotch, perhaps?) representing the nerve net and purple (a drizzle of red wine reduction?) standing in for muscle. Vanilla represents the mesoglea, a mostly cell-free aqueous gel that gives the beast some structure. The final chocolaty layer is the endoderm, which lines the jelly’s various internal cavities, including the pharynx. Please note that I don't actually know what any of these structures look like, so don't go eat a comb jelly and expect it to be a creamy, non-traumatic experience.

Ctenophores, despite being totally bereft of a brain, are actually pretty good at moving around in and reacting to their environment; gently cruising the ocean and snatching up unfortunate sea-critters with their inscrutable tentacles. They accomplish this feat by using their nerve net to both process signals from their sensory organs and coordinate the actions of their comb rows and muscle tissue.

Although this biological circuitry is about as powerful as a digital watch (and not the kind with a built-in calculator), it nonetheless makes comb jellies far more intelligent that the most scholarly and erudite sponge. Most species possess a single pair of feeding tentacles, which protrude laterally from the body and can be retracted into protective sheaths. The tentacles are covered colloblasts, a specialized type of cell that releases a gooey adhesive to adhere to prey items upon contact.

A cross-section of a single colloblast embedded in a Ctenophore's ectoderm. The reddish spheres at the top contain an adhesive that is released on contact. The coiled gray structure probably acts like a cellular shock absorber to protect the jelly’s tentacle as the prey helplessly thrashes and convulses in anguish. Ctenophores simply love killing.

Captured food is reeled in and licked off the tentacles by the jelly’s endoderm-lined mouth, where it passes into the muscular pharynx to be crushed and digested into a nutritive slurry which then flows into the beast’s stomachy canal system to be absorbed. These cilia-lined canals branch and radiate throughout the Ctenophore’s body (especially under the ctenes; see unwieldy cross-section above), allowing the rich broth to fuel its various tissues and functions, or be stored in elongated vacuole cells. Despite possessing several perfectly good anuses (=D), the jelly expels most of its metabolic waste by regurgitating it through the mouth (D=).

Sorry folks, I just couldn't bring myself to draw a detailed Ctenophore anus. Maybe next time.

While we’re on the subject of anuses, it’s worth mentioning that the sex lives of Ctenophores are rather tame; nearly all are hermaphroditic (and most of these are believed to be self fertile) and simply expel reproductive cells haphazardly into the surrounding water column in hopes of making more tiny jellies. A few species retain the developing Ctenophore larva in their endoderm, releasing them after a short gestation period. The juvenile jellies, called cydippids, are usually just miniature versions of the adults that gradually grow in size until they reach sexy-time maturity.

A larval Ctenophore, or cydippid. I usually just call them jelly kittens.

So, are you tired of comb jellies yet? No?

Good, me neither! In the next installment of APIGttTOL, we’ll explore Ctenophore diversity and evolution. Until then, Dear Reader, stay curious and rather gooey.

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2 thoughts on “A Poorly Illustrated Guide to the Tree of Life: Part 3”

Where do these guys, and I use the term “guys,” loosely fit in phylogenetically? I was originally taught they were a sister taxon to the Cnidarians, at a later date I saw a figure that placed them as deuterostomes. Where is literature standing now, as well were they originally placed (incorrectly) in either of those groups? I can see the radial symmetry confusing the placement but isn’t the development more decisive?